1. Field of the Invention
The present invention relates to the field of power steering systems for motor vehicles.
2. Prior Art
There are three primary types of power steering systems, hydraulic power steering systems, electro-hydraulic power steering systems and electric power steering systems. In a hydraulic power steering system, an engine driven fixed displacement pump is used with a flow control (pressure limiting or relief) valve for moderating system flow and pressure, and a rotary valve for distributing flow to and from the two sides of the rack piston. The rotary valve is responsive to the deflection of a torsion bar type of device on the steering shaft to sense the relative angular deflection between inner and outer valve members. The pump is sized to provide adequate power steering (adequate fluid flow) at engine idle speeds, with the flow through the rotary valve at all engine speeds being maintained at a constant level.
In an electro-hydraulic power steering system, a variable speed electric pump is used for flow control, with a rotary valve of the type used in a hydraulic power steering system being used for distributing flow to and from the two sides of the rack piston.
In a hydraulic power steering system, the pump is oversized for most operating conditions, leading to the excess of power steering fluid flow being dumped by the flow control valve, which wastes most of the pump energy. Also the rotary valve has a substantial short circuit from supply to vent when no power steering force is needed, thus also wasting pump energy. The rack, and thus pump outlet, always has a system-set pressure, even when no steering assist is needed. Thus the pump has to work against this pressure, again wasting pumping energy that is not used for steering assist.
In an electro-hydraulic power steering system, a rotary valve is still employed that also has a substantial short circuit from supply to vent when power assist is not needed, thus wasting pump energy any time steering assist is not active. This system also requires an electric pump, and thus a higher output electrical system on the vehicle.
An electric power steering system requires electric motors for assisting rack motion, and thus also a higher output electrical system on vehicle. It has also been limited to use in smaller vehicles, unless the vehicle electrical system is a full 42 Volt system.